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 Mod for fender rumble 100w combo bass amp
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jake-O.W.A. Offline





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PostPosted: Mon Oct 27, 2008 7:49 am    Post subject: Mod for fender rumble 100w combo bass amp Reply with quote

The amp has a built in compressor that really sounds dull and crappy. I would just like to add a bypass switch. Seems like it should be easy enough. I looked a bit for someone who has done this mod and I can't seem to find anything.

Anyone know the technical breakdown on this? How do I identify the compressor circuit? What's the right way to add the bypass?

Anyone?

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Lt. Bob Offline
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PostPosted: Mon Oct 27, 2008 8:30 am    Post subject: Reply with quote

Can't you just turn it off or is it one of those stupid designs where it's always in the circuit?
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skod Offline
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PostPosted: Mon Oct 27, 2008 8:40 am    Post subject: Reply with quote

Do you have a link to the schematic? I'd be happy to take a look and see if I could help out a bit...
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jake-O.W.A. Offline





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PostPosted: Mon Oct 27, 2008 9:06 am    Post subject: Reply with quote

Lt. Bob wrote:
Can't you just turn it off or is it one of those stupid designs where it's always in the circuit?
Yes.
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jake-O.W.A. Offline





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PostPosted: Mon Oct 27, 2008 9:07 am    Post subject: Reply with quote

skod wrote:
Do you have a link to the schematic? I'd be happy to take a look and see if I could help out a bit...
I can't for the life of me find it. Anywhere.

I will open it up and take pics if that'd help. Neutral

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skod Offline
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PostPosted: Mon Oct 27, 2008 9:12 am    Post subject: Reply with quote

What's the exact model name/number for the amp? I'll give a look and see what is around...
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jake-O.W.A. Offline





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PostPosted: Mon Oct 27, 2008 9:20 am    Post subject: Reply with quote

Fender Rumble 100 15"

http://bass-guitars.musiciansfriend.com/product/Fender-Rumble-100-Bass-Combo?sku=480721&src=3WFRWXX&ZYXSEM=0&CAWELAID=26039746

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skod Offline
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PostPosted: Mon Oct 27, 2008 10:00 am    Post subject: Reply with quote

The schematic is at http://www.manualnguide.com/dl/10215/ ...
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PostPosted: Mon Oct 27, 2008 10:10 am    Post subject: Reply with quote

Thanks a lot man! Can you tell from that where the compressor circuit is and could you make this really easy and show me where the bypass should go? Embarassed Laughing
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skod Offline
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PostPosted: Mon Oct 27, 2008 10:21 am    Post subject: Reply with quote

That is one freakin' nasty "compressor", as near as I can see: there are no threshold/slope/attack/release controls, right? So there's no variable gain cell anywhere, or anything like it. Yucko.

I think that what they are calling the "compressor" is right at the power amp input stage on sheet 3: the back-to-back 1N4448 diodes CR24 and CR25 on the grid input to V7B. They will clip the snot out of the signal once the swing exceeds their forward voltage, 600mV. That's just *asinine*... The "compressor" affects both input channels, right? Then that has to be it.

Try cutting the trace that couples the node between the diodes to the node between the grid input and the 470kOhm grid load resistor R1, and putting a simple little SPST switch in between them, to allow you to selectively hang the diodes on the grid circuit, or cut them loose. When closed, you get this clipping, and when open, you don't.

Gnarly. The intrinsic reverse-bias capacitance of those diodes is enough to put a nice highpass filter on the signal, in that somewhat high-impedance grid circuit- even (or perhaps especially!) when running below the actual clip threshold. I'll guarantee that mod will brighten up the tone a tad bit, if nothing else...

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jake-O.W.A. Offline





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PostPosted: Mon Oct 27, 2008 10:47 am    Post subject: Reply with quote

That's exactly what I am hoping. It is a delta comp limiter from their description.

So, if I take a shot of the circuit before I cut stuff can you take a look at it for me and ok it?

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skod Offline
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PostPosted: Mon Oct 27, 2008 11:13 am    Post subject: Reply with quote

Well, that sure as hell is a _limiter_... (;-)

Sure- take a pic. The good news is that they have a good component identification sheet there. So, rather than actually cutting a trace, you could just lift the diodes, tie them together, and put the switch in series to the hole where one of them used to be. That way you could easily put the thing back to its original condition if you wanted to...

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PostPosted: Mon Oct 27, 2008 11:56 am    Post subject: Reply with quote

Awesome. So let me get this straight, basically I am just breaking the circuit? Don't I need to have another wire go bypass the circuit? I want to mount a switch in the back of the amp and make it permanent. Not sure why I would ever want to reengage it but who knows? Maybe they put it there to protect their crappy speaker design from spikes. If I plug a preamp into the amp I would still like to have the option if I want it.
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skod Offline
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PostPosted: Mon Oct 27, 2008 12:16 pm    Post subject: Reply with quote

jake-O.W.A. wrote:
So let me get this straight, basically I am just breaking the circuit? Don't I need to have another wire go bypass the circuit?


No, you actually aren't breaking the circuit- the original signal path will still be intact. All you are doing is removing a nonlinear load that drags the signal down right at the input to the power amp stage. The diodes just shunt some of the incoming signal to ground when it gets above a certain level.

jake-O.W.A. wrote:
I want to mount a switch in the back of the amp and make it permanent. Not sure why I would ever want to reengage it but who knows? Maybe they put it there to protect their crappy speaker design from spikes. If I plug a preamp into the amp I would still like to have the option if I want it.


Shoot, if you want to make it *permanent*, just desolder those two diodes completely: no switch really needed. You could always solder them back in if you wanted to restore the amp to original at some point (perhaps to sell it?). If you wanted to allow for that, you could just tape them to the inside of the chassis somewhere so you wouldn't lose them. You wouldn't even have to drill a hole for the switch at that point...

That is a damned lousy limiting technique- nasty as all get-out. I don't think you'll miss it overmuch...

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PostPosted: Mon Oct 27, 2008 12:24 pm    Post subject: Reply with quote

Oh man, easy! Thanks Skod, I will let you know how it turns out.
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Lt. Bob Offline
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PostPosted: Mon Oct 27, 2008 12:53 pm    Post subject: Reply with quote

jake ..... you needin' a bit more amp for your bass?
I may have a head you could have if you wanted.

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PostPosted: Mon Oct 27, 2008 2:32 pm    Post subject: Reply with quote

Lt. Bob wrote:
jake ..... you needin' a bit more amp for your bass?
I may have a head you could have if you wanted.
Holy shit man, I sure am. Problem is I just wasted $330 on this hunk of junk and I don't have much in the way of a cabinet. Going into winter I wanted to have something that could keep up with my drumming down here but going into winter is my hardest time financially.

I have wired my old Vox t25 as a monitor by the drums but that just distorts and freaks out.

What kind of head is it? I could probably pay you by summer next year! Embarassed Laughing

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PostPosted: Mon Oct 27, 2008 2:32 pm    Post subject: Reply with quote

But besides all that, this limiter just sounds like shit, I didn't realize it didn't have a defeat switch so this will help.

Hey Skod, will plugging into the "pwr amp in" of the amp bypass the limiter? It sounds like it would from what you way but I don't know if it wires back into the circuit from that jack or what.

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PostPosted: Mon Oct 27, 2008 2:35 pm    Post subject: Reply with quote

It's messed up because this bass amp sounds amazing as a guitar amp with my tone port on the frontside.
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skod Offline
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PostPosted: Mon Oct 27, 2008 2:50 pm    Post subject: Reply with quote

jake-O.W.A. wrote:
Hey Skod, will plugging into the "pwr amp in" of the amp bypass the limiter? It sounds like it would from what you way but I don't know if it wires back into the circuit from that jack or what.


I don't even see the "power amp in" jack listed in the schematics- so I suspect that it won't bypass that. Those diodes are on the power amp board itself, not on the preamp board- so I suspect that the power amp in connection is just a normalling jack stuck in the wiring harness between the two boards. If that's the case, the first thing the signal will hit on its way into the power amp is that pair of diodes...

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PostPosted: Mon Oct 27, 2008 3:00 pm    Post subject: Reply with quote

That's what it sounds like is happening, thanks.
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Lt. Bob Offline
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PostPosted: Mon Oct 27, 2008 5:07 pm    Post subject: Reply with quote

I have an OLD Gallien Kruger 400B that you can have ..... just whatever shipping is unless you live near here and want to pick it up. Then it would be free.
NOTE: I have used this amp for maybe 20 years and it has always worked fine however .... it got all smokey from a housefire and looks like shit. It's only smoke damage and the amp has always worked fine ..... but you have to get a strong light and get up real close to read the lettering because it's all obscured by the smokey coloring the entire amp has. Very Happy
So I stuck a piece of adhesive tape under the knobs and wrote what each control is. It's only 200 watts or so into probably 4 ohms but it ought to outdo that Fender.
I just got my new SWR head and I've got too much crap so I was gonna give this away anyway. I figured I'd find some kid that needs one but I'd rather it went to you if you want it.

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PostPosted: Mon Oct 27, 2008 5:13 pm    Post subject: Reply with quote

I damn well do!

And thank you tons man. I will totally send you a check for shipping the second you tell me how much it is. Cool thing is I don't need a huge cabinet for that thing and I can probably clean it up nicely.

PM on the way!

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Lt. Bob Offline
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PostPosted: Mon Oct 27, 2008 9:09 pm    Post subject: Reply with quote

cool .... I'll get it off to ya' next week ..... I'm having a wisdom tooth out wed and I'm gonna be wussing it up!
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PostPosted: Mon Oct 27, 2008 9:32 pm    Post subject: Reply with quote

I hear you, I need to get one out as well.
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